<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>补0案例</title>
</head>

<body>
  <script>
    let num = +prompt('请输入一个数字')
    num = num >= 10 ? num : '0' + num
    console.log(num);
  </script>
</body>

</html>